Method for locking an artificial facet between two vertebral bodies

ABSTRACT

A bone screw ( 1 ) is used for locking an articular facet between the superior and inferior articular processes of two vertebral bodies and is provided with a threaded shaft ( 2 ), a screw head ( 3 ) and a central axis ( 4 ). The screw head ( 3 ) is further provided with a number of grooves ( 5 ) on the periphery of the screw head ( 3 ) which run essentially parallel to said central axis ( 4 ).

This application is a continuation of U.S. patent application Ser. No.11/126,976, filed May 10, 2005 now U.S. Pat. No. 7,699,878, which is acontinuation of International Patent Application No. PCT/CH2002/000608,filed Nov. 13, 2002, the entire contents of which are incorporatedherein by reference thereto.

This invention concerns a bone screw, in particular for locking anarticular facet between the superior and inferior articular processes oftwo vertebral bodies.

To achieve a circumferential arthrodesis (fusion) the anterior and theposterior columns must be treated. The goal of the treatment is therestoration of the lordotic curve and the anatomically correct discspace. Anteriorly, i.e. in the intervertebral space, implants like cagesare inserted after disc removal. The posterior vertebral column, wherethe articular facet is located, should be locked as well.State-of-the-art techniques consider translaminar screws or transpedicalinstrumentation which, however, are not satisfactory.

Translaminar screws have certain disadvantages, like

a) the screw insertion point is difficult to localize; the localizationis done under full view, i.e. a separate posterior incision is performedmedially whereas muscles must be separated from spinal and laminarprocesses. Most morbidity results from medial incision;

b) the surgeon's view is two-dimensional due to the small incision whichmay result in interference of the second screw with the first one; and

c) aiming devices or navigation tools did not improve insertiontechnique or precision of screw placement.

From U.S. Pat. No. 2001/007074 A1 STROBEL a bone screw is known.

From U.S. Pat. No. 4,754,749 TSOU another bone screw is known which hastwo closed canals in the screw head at an angle with the screw axis. Aguide pin is insertable in one of these dosed canals, so that uponinsertion of the guide pin it projects radially over the shaft andaxially over the head of the screw which renders it complicated andrather unstable.

The invention as claimed aims at solving the above described problems.

The present invention provides a bone screw and a method for locking anarticular facet of a vertebral body.

The advantages of said method and the use of said bone screw are thefollowing:

-   -   percutaneous approach which results in reduced morbidity;    -   low implant cost due to simple design, in particular in        comparison to transpedicular fixation;    -   lower risk with regard to screw positioning compared to        translaminar screws; and    -   easy removal of the screws.

The interference screw according to the invention allows a new surgicaltechnique to lock the articular facets of vertebral bodies.

The natural functional spine unit (FSU) contains two articular facets.The function of the screw according to the invention is the interferencein the sense of obstruction or fixation of said articular facets. Sincethe core diameter of the screw is significantly larger than the gap inthe articular facet, the device is hindering the natural articulation.The function of the screw thread is the insertion by rotation. Afterinsertion the screw thread protects the screw from axial migration andthe anti-rotation device protects the screw from migration by rotation.

According to the new surgical method the locking screws are insertedthrough two percutaneous approaches in the trajectories parallel to thearticular surface of the articular facet. Aiming wires guarantee thecorrect positioning. An anti-rotation element keeps the bone screw inposition and hinders the screws from turning out if micro-motion isapplied to the screws. This technique is applicable if the anteriorvertebral column (i.e. the intervertebral space of the related segment)is stabilized with a spacer such as an intervertebral cage.

According to a special embodiment the grooves on the periphery of thescrew head are running essentially parallel to the central axis. Thisshall be interpreted in such a way that minor angulation with regard tothe central axis either towards radially the latter or tangentially toit would still be functional. Such an angulation in a radial plane andmeasured relative to the central axis might be in the order of up to60°, but preferably lees than 20°. The possible angulation in atangential plane and measured relative to the central axis might be inthe order of maximum 20°, preferably less than 10°.

In a particular embodiment the bone screw is provided with at least onepair of diametrally opposed grooves on the periphery of said screw headwhich enhances stability of the implant.

The screw head may be provided with a central cavity coaxially arrangedwith respect to said central axis, e.g. with a polygonal profile,preferably a hexagonal profile for receiving a screw-driver having acorresponding profile.

The grooves on the periphery of the screw head may be juxtaposed to thepolygonal planes of said central cavity. By this measure a highermechanical strength can be achieved.

In a further embodiment an anti-rotation element is insertable in saidgroove or said pair of grooves on the periphery of said screw head,whereby said anti-rotation element in its inserted position projectsradially out of the periphery of said screw head. The anti-rotationelement is preferably a U-shaped staple with two legs and a centralportion bridging said two legs and designed for insertion into saidgroove or said pair of grooves of said screw head. The anti-rotationelement has preferably a diameter which is larger than said screw head.The anti-rotation element in form of a U-shaped staple may be providedwith a guiding element attached to said central portion and runningessentially parallel to said legs. The guiding element may be in theform of a plate, a circular cylinder or a prism designed for insertioninto said central cavity of said screw head. The cylindrical shape ofthe guiding element has the advantage of a more accurate gliding.

The central portion of the anti-rotation element may be provided with atleast one perforation for removal of the screw.

The threaded shaft of the bone screw has preferably a thread with a highangle of pressure, e.g. in the range of 4° to 70°. The flank of saidthread can be symmetrically or asymmetrically oriented. Theasymmetrically oriented thread is compressing particularly cancellousbone. This increases initial fixation stability.

The bone screw may be self-tapping, preferably by means of a cuttingedge.

The core of the screw shaft may be either cylindrical or tapering awayfrom the screw head.

In the case of cylindrical core of the screw shaft various advantagescan be achieved, namely:

-   -   a continuous bending strength along the screw shaft;    -   the application of a constant insertion torque while turning the        screw in the articular facet; and    -   due to the constant shaft diameter the screw does not become        loose if the device is turned back slightly after insertion.    -   This could appear if the surgeon brings the grooves for the        anti-rotation element in congruent direction like the articular        facet gap.

In the case of a conical shape of the core the shaft is compressing thesurrounding bone. This increases the initial stability of the implant.

The envelope of the threaded shaft may be cylindrical allowing also aconstant insertion torque. However, the envelope of the threaded shaftpreferably tapers away from the screw head so that the purchase of thethread in the bone is increasing by turning the screw in.

The bone screw may be self-drilling, preferably by means of a chuckinggroove.

The new method for locking an articular facet between the superior andinferior articular processes of two vertebral bodies consists in theinsertion of the threaded shaft of a bone screw in the gap of saidarticular facet. To that purpose the bone screw is preferably cannulatedand insertion is performed by means of an aiming wire. The bone screwhas preferably a screw head with a larger diameter than said threadedshaft and said threaded shaft is inserted in said gap of said articularfacet until said screw head touches the bone. Upon insertion of saidbone screw an anti-rotation element may be applied to said screw headsuch that rotation of said bone screw is prevented.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming partof this disclosure. For the better understanding of the invention, itsoperating advantages and specific objects attained by its use, referenceshould be had to the accompanying drawings, examples and descriptivematter in which are illustrated and described preferred embodiments ofthe invention.

IN THE DRAWINGS

FIG. 1 is a perspective view of the bone screw according to theinvention together with an anti-rotation element to be used with thescrew;

FIG. 2 a is a longitudinal section through the central axis of the screwand the anti-rotation element according to FIG. 1;

FIG. 2 b is a top view of the bone screw according to FIG. 1 with theanti-rotation element inserted into the screw head;

FIG. 3 is a perspective view of a trocar for bringing an aiming wireinto the gap between the articular facet;

FIG. 4. is a perspective view of a drill bit, the aiming wires beingtemporarily fixed in the gaps of the articular facets;

FIG. 5 is a perspective view of the bone screw being inserted by meansof a screw-driver into the gap of the articular facet by using thetemporarily fixed guiding wire;

FIG. 6 is a perspective view of the anti-rotation element being put overthe screw head into its grooves closest to the joint gap; and

FIG. 7 is a perspective view of the inserted bone screw to which theanti-rotation element has been attached.

The bone screw 1 as represented in FIGS. 1, 2 a and 2 b is used inparticular for locking an articular facet between the superior andinferior articular processes of two vertebral bodies. It has a threadedshaft 2, a screw head 3 and a central axis 4. The screw head 3 isprovided with six grooves 5 regularly disposed on the periphery of thescrew head 3 running essentially parallel to the central axis 4. Thescrew head 3 is further provided with a central cavity 6 coaxiallyarranged with respect to the central axis 4 and having a hexagonalshape.

The anti-rotation element 10 is provided with a U-shaped staple havingtwo legs 11 and a central portion 13 bridging said two legs 11. TheU-shaped staple is provided with a guiding element 12—having the shapeof a circular cylinder—attached to the central portion 13 and runningessentially parallel to the legs 11. As can be seen in FIGS. 1 and 2 athe anti-rotation element 10 may be connected to the bone screw 1 bymoving it along the central axis 4 whereby its central portion 13 entersthe central cavity 6 of the screw head 3 and the two legs 11 areinserted into one of the three pairs of grooves 5 of the screw head 3 asshown in FIG. 2 b.

The free ends of the two legs 11 are provided with an protrusion 15oriented radially inwards to the central axis 4 so that when the legs 11are gliding along the grooves 5 the protrusions 15 will click under thelower edge 16 of the screw head 3 thereby securing the anti-rotationelement 10 against withdrawal in the opposite axial direction.

The central portion 13 of the U-shaped staple is further provided withat a perforation 14 facilitating removal of the bone screw 1.

Useful materials for the bone screw 1 as well as for the anti-rotationelement 10 are titanium, titanium alloys or fiber-reinforced plasticmaterials. They may be coated with ceramic.

A detailed method of operation follows for the better under-standing ofthe invention:

1. Both positions in extension of the articulating planes of thearticular facet concerned are identified and marked accordingly on theskin. To this purpose an image intensifier is used to control positionand direction.

2. Bilateral skin incisions are performed in the direction of thearticular facets.

3. Depending on the surgeon's preference, a trocar 17 or similarinstrument is used to bring an aiming wire 18 into the gap 19 betweenthe articular facets (FIG. 3). Positional control is indicated using animage intensifier.

4. As shown in FIG. 4 the aiming wires 18 are temporarily fixed in thegaps 19 of the articular facets by means of the drill bit 20 (“screwhead reamer” or “counter sink”).

5. As shown in FIG. 5 a cannulated and self-tapping bone screw 1 isinserted by means of a screw-driver 21 into the gap 19 by using thetemporarily fixed guiding wires 18 until the screw heads 3 of the bonescrews 1 are touching the bone.

6. As shown in FIG. 6 an anti-rotations element 10 is put over the screwhead 3 in the grooves 5 (notches) closest to the joint gap 19.Eventually the bone screw 1 must be turned back by some degrees in orderto match the grooves 5 (notches) in the screw head 3 with the joint gap19 (FIG. 7).

7. All instruments are removed.

8. The placement of the bone screws 1 is verified by using an imageidentifier and the wound is closed.

The method of operation as described can be performed on one side of thevertebral column only but is preferably performed simultaneously on theright and left side, as shown in FIGS. 3 to 7, which has biomechanicaladvantages.

1. A method of inserting an intra-facet bone screw into a facet jointbetween a superior articular surface of an inferior vertebral body andan inferior articular surface of a superior vertebral body, said methodcomprising: inserting a threaded shaft of a bone screw into the facetjoint between the superior articular surface of the inferior vertebralbody and the inferior articular surface of the superior vertebra so thatthe threaded shaft of the bone screw partially engages the superior andinferior articular surfaces, a longitudinal axis of the bone screw beingsubstantially aligned with and located between the superior articularsurface of the inferior vertebral body and the inferior articularsurface of the superior vertebral body.
 2. The method of claim 1,wherein the bone screw is cannulated; and said inserting comprisesinserting said threaded shaft of said bone screw into the facet jointusing an aiming wire.
 3. The method of claim 1, wherein: said bone screwhas a screw head with a larger diameter than said threaded shaft; andsaid inserting comprises inserting said threaded shaft of said bonescrew into the facet joint until said screw head touches bone.
 4. Themethod of claim 1, wherein said inserting is performed on right and leftsides of a vertebral column in a single surgical procedure.
 5. A methodof inserting an intra-facet bone screw into an articular facet joint gapbetween superior and inferior articular processes of two vertebralbodies of a patient's spine, the method comprising the steps of: a)inserting a portion of an aiming wire into the articular facet jointgap; b) inserting the intra-facet bone screw into the facet joint gap,the intra-facet bone screw being inserted into the facet joint gapbetween the superior articular process of an inferior vertebra and theinferior articular process of a superior vertebra such that anexternally threaded shaft of the intra-facet bone screw partiallyengages the superior articular process of the inferior vertebra and theinferior articular process of the superior vertebra, a longitudinal axisof the intra-facet bone screw being substantially aligned with andlocated between the superior articular process of the inferior vertebraand the inferior articular process of the superior vertebra; and c)removing the aiming wire from the facet joint gap.
 6. The method ofclaim 5, wherein the intra-facet bone screw is percutaneously insertedinto the facet joint gap in step (b).
 7. The method of claim 5, furthercomprising the step of: d) placing a mark on a patient's skin toidentify an articulating plane of the facet joint gap; and e) forming abilateral skin incision at the mark.
 8. The method of claim 5 comprisingthe further step of: d) inserting a trocar at least partially into thefacet joint gap prior to step (a).
 9. The method of claim 5 wherein thefacet joint gap is prepared prior to step (a) with a drill bit.
 10. Themethod of claim 5, wherein the intra-facet bone screw is inserted instep (b) until a head portion of the intra-facet bone screw touches boneadjacent the facet joint gap.
 11. The method of claim 10 comprising thefurther step of: (d) placing an anti-rotation element over the headportion of the intra-facet bone screw.
 12. The method of claim 5comprising the further step of: (d) verifying placement of theintra-facet bone screw with an image identifier prior to step (c). 13.The method of claim 5, wherein said inserting is performed on right andleft sides of a vertebral column in a single surgical procedure.